It is known, particularly in oil exploration, to determine the position of oil reservoirs from the results of seismic measurements performed from the surface or in wells. In the reflection seismology technique, these measurements involve emitting a wave into the subsoil and measuring a signal including various reflections of the wave on the geological structures looked for. These structures are typically surfaces separating distinct materials, faults . . .
The measurements are processed for building a model of the subsoil, in general in the form of seismic images. These images may be two-dimensional (seismic sections) or three-dimensional (seismic blocks). A seismic image is composed of pixels whose intensity is representative of a seismic amplitude depending on local variations of the impedance. The geophysicists are used to analyzing such seismic images. Through visual observation, they can distinguish areas of the subsoil having different features in view of determining the structure of the subsoil.
For offshore exploration, there is generally use of hydrophones distributed along receiver lines pulled by vessels and a source such as a compressed air gun to emit seismic waves in the marine environment.
In desert or easy access plain areas, receiver lines are used, along which geophones are arranged, and the shots are generally performed with vibrating sources carried by special vehicles moving in the studied area.
In mountainous or foothill regions that are inaccessible to the vibrators, the shots are performed using explosives transported by men or by helicopter to the desired locations.
In terrestrial environments, it is necessary to prepare the site in order to install receiver lines. Most often, the geophones are buried and linked to each other via cable networks transporting the signals useful for acquiring data. It is also possible to use geophones operating with a wireless station sharing the synchronization information via radio. The implementation of the sources also requires a preparation of the site so as to allow the burial of the explosives or the circulation of the vibrator trucks. Once the measurements are completed, the lines are dismantled and the site must be returned to its initial condition. These field operations contribute significantly to the complexity and cost of the exploration. In desert areas these constraints remain manageable. However, when it is desired to explore the subsoil of regions where access is more difficult or where the ground presents relief and/or vegetation, in particular in mountainous or foothill regions, the cost of a measurement campaign, related to the arrangement of the receiver lines, to the transport or installation of the seismic sources, to the preparation and return to the initial condition of the site can become very significant, if not prohibitive.
It is possible to limit the cost of the exploration procedure by reducing the spatial density of the shooting positions of the receiver positions. However, this degrades the quality of the seismic images obtained due to a reduced spatial sampling.
In orthogonal acquisition geometries of relatively low (“sparse”) density for producing three-dimensional seismic imaging (3D), the shots and receivers are located at positions that are relatively close to each other along the individual lines, e.g. a few tens of meters, whereas the distance between these lines is relatively large, e.g. in the order of 1 km. The line interval governs the seismic fold. This seismic fold, corresponding to the number of times a given zone of the subsoil is exposed by the emitted seismic waves, decreases when the line interval increases. The fold resulting from these sparse geometries is poor at small and medium depths. Combined with the strong heterogeneities of the speed close to the surface in mountainous areas, this poor fold leads to low-quality seismic data, at small and medium depths, the measured signal being dominated by high order reverberations, scattering, volume wave-surface wave couplings. Such conventional sparse geometries are suitable mainly for deep exploration, but give bad results for representing shallow structures.
When the orthogonal geometry is too sparse, the fold is not optimal at small or medium depth, and gives rise to artifacts that cannot be properly attenuated by the migration technique, even in the ideal case where the model of the subsoil would be perfectly known for the imaging.
There is therefore a need to improve the 3D seismic imaging techniques using relatively sparse imaging geometries.